A continuously variable automatic transmission (CVT) usually consists, among others, of a starting unit, a forward/reverse drive unit, an intermediate shaft, a differential, hydraulic and electronic control devices and one variator. A customary variator construction is a belt-drive variator, having a primary and a secondary pulley, wherein both pulleys are formed by beveled pulleys disposed in pairs and, wherein the variator is provided with a torque transmitting belt-drive variator, having a primary and a secondary pulley, wherein both pulleys are formed by beveled pulleys disposed in pairs and, wherein the variator is provided with a torque transmitting belt-drive element which rotates between the two beveled pulley pairs. In such a transmission, the actual ratio is defined by the running radius of the belt-drive element which, in turn, is function of the axial position of the beveled pulleys. A CVT is consequently a positive-engagement transmission; the engine torque is transmitted by (static) friction between the belt-drive element and the variator pulleys.
In the prior art, the variator is mostly functionally divided in one side for control of the change of ratio (adjustment) and, in other side, for adjustment of the necessary contact pressures (tightening) which ensure the required contact between the pulley and the belt-drive element so that no slip generates. Depending on the operating point, either the primary or the secondary pulley is used for setting the contact pressures.
For this purpose, the transmission control adjusts on the variator a hydraulic contact pressure in order to make the momentary transmission of the engine torque possible. The main input variables for the pressure are the momentary transmission input torque and the transmission ratio. The contact pressure must be as high as needed and as low as possible; if the pressure is too low, the slipping of the variator (slip) results and therewith a damage to the transmission. If the pressure is too high, the efficiency of the transmission is unnecessarily impaired.
The contact pressure requirement is affected by many parameters not predictable at the time such as oil aging, smoothing of the pulley surface or kind of oil.
To prevent slippage between the pulley and the belt-drive element, the teaching of the prior art is to impose increased security or security factors on the calculated control pressure values. This procedure has the disadvantage that thereby the efficiency of the automatic transmission is impaired. Therefore, the increased security (security factor) must be selected as small as possible.
Accordingly, the problem on which this invention is based is to outline, departing from the cited prior art, a process for variator slip treatment and for protection of the variator for a CVT so as to obtain the best possible efficiency of the transmission and, at the same time, improve the prevention of damages to the transmission.
The inventive process must adapt the contact pressure to the operating point position so that it becomes possible to reduce the effects of an overpressure to a minimum which affects the efficiency.
According to the invention, this problem is solved with the features of claim 1. Other embodiments of the invention result form the sub-claims.
Accordingly, when the variator slip is detected, it is proposed locally to increment in the place where the slip occurs a slip meter characteristic field depending on the ratio value iv of the variator and on the engine torque M_mot f (iv, M_mot) and at the same time increase, via an evaluation module, an evaluated slip meter, the variator slip being assessed by means of the evaluation module according to duration and intensity.
It is further proposed to add to the evaluated meter a characteristic field of slip-status meter which is incremented with the evaluated slip meter.
Parallel to incrementing the meters, a belt slip error is entered in a diagnosis or error memory so that with the entry of the error, slip, evaluated slip and slip-status meter are entered as environmental conditions in the diagnosis or error memory.
According to the invention, unlike the belt-slip meter, the status meter can be locally decremented via a xe2x80x9cself-healing functionxe2x80x9d when the transmission behavior is perfect with regard to the variator slip.
The status meter serves as factor for locally lifting the contact pressure security f(iv, M_mot) whereby in case of unnecessary slip detection or self-healing processes in the transmission mechanics (e.g. smoothing of the pulleys in the belt-drive variator), the contact pressure security can be again withdrawn which results in improvement of the efficiency.
The whole contact pressure security also results from other security factors to be taken into account. If the total contact pressure security exceeds a specific level (e.g. system limit or breaking limit), then a second error information is entered in the diagnosis memory (error memory), which information can be reacted to with other substitute function (e.g. hydraulic emergency running).
By deletion of the diagnosis or error memory, the error entries can be deleted but not belt slip meter and status meter. These are independent of the error memory and can be separately deleted. A resetting of the whole function by deleting the transmission diagnosis is thereby eliminated.
The switch for resetting slip and slip status meter must be provided with greater securities (qualifications) than the deletion of the diagnosis memory.